#include<bits/stdc++.h>
using namespace std;
using ll=long long;



int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    
    int N,M;
    cin>>N>>M;
    vector<string> S(N);
    for(int i=0;i<N;i++)cin>>S[i];
    string AN="";
    vector<bool> E(N,1);
    for(int i=0;i<M;i++){
        set<char> C;
        for(int n=0;n<N;n++){
            if(!E[n])continue;
            C.insert(S[n][i]);
        }
        if(C.size()==3){
            cout<<"-1"<<endl;
            return 0;
        }
        else if(C.size()==2){
            if(!C.count('C'))AN.push_back('P');
            if(!C.count('G'))AN.push_back('C');
            if(!C.count('P'))AN.push_back('G');
            for(int n=0;n<N;n++){
                if(AN.back()!=S[n][i])E[n]=0;
            }
        }
        else{
            if(C.count('C'))AN.push_back('G');
            if(C.count('G'))AN.push_back('P');
            if(C.count('P'))AN.push_back('C');
            while(AN.size()<M)AN.push_back('C');
            cout<<AN<<endl;
            return 0;
        }
    }
    for(int i=0;i<N;i++){
        if(E[i]){
            cout<<-1<<endl;
            return 0;
        }
    }
    cout<<AN<<endl;
}